Rubber compounds for various tire components (e.g., tire treads) typically contain particulate reinforcing fillers. While traditional methods have used carbon black as filler, silica fillers are becoming more prevalent in tire compounds due to their improved tradeoffs in stiffness, hysteresis, and wet skid resistance. Additionally, silica fillers can reduce the rolling resistance of tires, thereby potentially increasing fuel economy.
To optimize reinforcement properties, reinforcing filler should be capable of being finely divided and homogeneously distributed throughout the elastomeric matrix. While the traditional carbon black filler has both of these capabilities, silica filler tends to agglomerate due to hydrogen bonding between silanol groups on the surface of the silica particles. This agglomeration leads to poor silica dispersion within the elastomeric matrix and tends to yield a rubber compound with undesirable performance characteristics. To minimize this problem, silica can be dispersed by the addition, for example, of a bi-functional silane coupling agent, or a mixture of such agents, that have one moiety (e.g., a silyl group) which reacts with the silanol groups on the surface of the silica particles and another moiety (e.g., a polysulfide group) that binds the silica particles to the elastomer upon vulcanization. Such bi-functional silane coupling agents are known to those having ordinary skill in the art.
Although these silane coupling agents tend to produce adequate dispersion of the silica filler throughout the elastomeric matrix, there are at least two disadvantages when using these agents. First, silane coupling agents are expensive relative to most other rubber compound ingredients. Second, a relatively large amount of the silane coupling agent, sometimes as much as 20% by weight based on the weight of the silica filler, is required to obtain adequate silica dispersion. Together, these two disadvantages substantially increase the cost of the resulting rubber compound used in the tire.
It would therefore be desirable to provide a rubber compound that includes silica filler and a silane coupling agent wherein the amount of silane coupling agent can be reduced without substantially sacrificing the performance characteristics of the rubber compound.